Current-apertured vertical cavity laser

ABSTRACT

A vertical-cavity surface-emitting laser (VCSEL) has an active region, first and second mirror stacks forming a resonant cavity with a radial variation in index forming a transverse optical mode, and a thin insulating slot within the cavity to constrict the current to a diameter less than the beam waist of the optical mode thereby improving device efficiency and preferentially supporting single mode operation. In one embodiment, an insulating slot is formed by etching or selectively oxidizing a thin aluminum-containing semiconductor layer in towards the center of a cylindrical mesa. The slot thickness is sufficiently thin that the large index discontinuity has little effect on the transverse optical-mode pattern. The slot may be placed near an axial standing-wave null to minimize the perturbation of the index discontinuity and allow the use of thicker slots. In a preferred embodiment, the current constriction, formed by the insulating slot, is located on the p-type side of the active region and has a diameter significantly less than the beam waist of the optical mode, thus minimizing outward diffusion of carriers and ensuring single transverse-mode operation of the laser by suppressing spatial hole burning.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor vertical-cavity surface-emittinglasers (VCSELs), and more particularly to structures and techniques forproviding highly-efficient and single-mode VCSELs. A VCSEL is asemiconductor laser consisting of a semiconductor layer of opticallyactive material, such as gallium arsenide or indium gallium arsenide orthe like, sandwiched between highly-reflective layers of metallicmaterial, dielectric material, epitaxially-grown semiconductordielectric material or combinations thereof, most frequently in stacksknown as distributed Bragg reflectors. As is conventional, one of themirror stacks is partially reflective so as to pass a portion of thecoherent light built up in the resonating cavity formed by the mirrorstack/active layer sandwich.

Laser structures always require optical confinement and carrierconfinement to achieve efficient conversion of pumping electrons tostimulated photons. Carrier confinement is generally achieved by varyingthe resistivity of the materials between the electrical contacts and theactive region. Such techniques include introduction of high resistivitythrough ion bombardment, variations in doping and dopant type, removalof conductive material by etching as well as conversion of semiconductorinto insulating oxide by selective oxidation. Optical confinement isachieved by varying the index of refraction of the materials in thestructure. It is convenient to discuss the cavity in terms of acylindrical geometry, although many cross sections are possible and theinvention is not limited to cylindrical geometries. The axial mode'sstanding-wave pattern in a VCSEL is very strong due to the highreflectivity of the mirrors, typically in excess of 99%. The relativelyshort optical path length between the two mirrors results in arelatively large wavelength separation between resonant axial modes. Thelarge wavelength separation ensures that the VCSEL lases in only asingle axial mode. The transverse or radial optical mode's intensityprofile is determined by radial index variations in the cavity. Adesirable mode is the fundamental mode, for example the HE₁₁ mode of acylindrical waveguide. A fundamental mode signal from a VCSEL is easy tocouple into an optical fiber, has low divergence, maintains a stable farfield pattern and is inherently single frequency in operation.

VCSELs which introduce radial index variations for optical confinementare known as index-guided VCSELs. In a conventional edge-emittingsemiconductor laser, the transverse index guide (perpendicular to thegrowth direction) is designed for an index variation on the order of0.1% to achieve single transverse mode operation. The small indexvariation precludes the existence of higher order transverse modes.Regrown or ridge waveguide structures are typically used inedge-emitting lasers to form the transverse index guide. In order for aVCSEL to lase, the mirror reflectivities in the axial direction must bevery high and thus the layers must be epitaxially grown or depositedwith a high degree of planarity. The requirement of high planarity hasprecluded the effective use of regrowth or ridge waveguide designs inVCSELs.

The previously-known index guided VCSELs use either etched-post orinsulating slots to provide carrier confinement and introduce a radialindex variation. Unfortunately, the radial index variation is relativelylarge and results in a cavity which supports multiple transverse modes.Furthermore, the carrier confinement is generally at a diameter equal toor larger than the transverse optical-mode diameter. As the optical modeis weak at the edges, conversion of the carriers into light bystimulated emission is reduced substantially for those carriers in theactive region outside the characteristic diameter (or beam waist) of thetransverse optical mode.

What is needed is a VCSEL structure that provides carrier confinement toa diameter less than that of the transverse optical mode and yet can berealized while maintaining a high degree of planarity in the layers. Inco-assigned U.S. Pat. No. 5,343,487, the inventors disclosed a VCSELthat provided a constriction of carriers to an aperture less than thetransverse optical mode. The disclosed laser used a ring-contactgeometry so that ion implantation or diffusion techniques could be usedto alter the conductivity of a semiconductor layer, providing a currentconstriction without introducing an optical constriction. The earlierpatent also disclosed a resistive current-leveling layer between theactive region and the conductive layer to minimize current-crowdingeffects associated with ring-contacted junctions. The drawbacks of thislaser are the complexity of fabricating a ring-contacted geometry andthe difficulties of using the semiconductor altering techniques, such asion implantation and diffusion, that do not introduce significantrefractive-index discontinuities.

It would be highly desirable to be able to introduce a currentconstriction with technologically-simpler techniques that introducerefractive-index discontinuities without constricting the transverseoptical mode as well. The present invention allows such simplertechniques for current construction to be used without constricting theoptical mode, and therefore the present invention has broadapplicability to a wide variety of VCSEL structures.

SUMMARY OF THE INVENTION

According to the invention, in a vertical-cavity surface-emitting laser(VCSEL) with an active region, first and second mirror stacks forming aresonant cavity, and a radially-varying index profile which defines thetransverse mode, an electrically insulating slot (or slots) isintroduced within the cavity to constrict the current to a diameter lessthan the beam waist of the optical mode. According to the invention, theinsulating slot is designed to be so thin that the effective indexdiscontinuity does not constrict the optical mode. In a preferredembodiment, the insulating slot is placed near a null in the axialmode's standing-wave pattern allowing for thicker slots withoutconstricting the optical mode.

The invention will be better understood upon reference to the detaileddescription in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of a prior art Vertical-CavitySurface-Emitting Laser (VCSEL) illustrating the transverse optical modeand current flow.

FIG. 2 is a side cross-sectional view of a different prior art VCSELillustrating the transverse optical mode and current flow.

FIG. 3 is a side cross-sectional view of a VCSEL according to theinvention, illustrating the transverse optical mode.

FIG. 4 is a detail side cross-sectional view of the device of FIG. 3illustrating the current flow.

FIG. 5 is a detail axial cross-sectional view of the device of FIG. 3illustrating the relation between the axial optical mode and the layers.

FIG. 6 is the reflectivity spectrum for the two radial regionsidentified in FIG. 4, showing the respective cavity mode resonances.

FIG. 7 is a plot of the transverse mode diameter as a function of theshift in axial cavity-mode resonance between the two regions.

FIG. 8 is a plot of the shifts in axial cavity-mode resonance forvarying insulating-slot thickness. One curve is for the slot centered onan axial standing-wave null, the other for the slot centered on an axialstanding-wave peak.

FIG. 9 is a side cross-sectional view of a detail of another VCSELaccording to the invention, using a thick slot to confine the mode and athin slot to confine the current.

FIG. 10 is a side cross-sectional view of a detail of another VCSELaccording to the invention, showing a pair of thin insulating slots oneither side of the active region.

FIG. 11 is a side cross-sectional view of a detail of another VCSELaccording to the invention, showing the thin slot combined with thethick slot.

FIG. 12 is a side cross-sectional view of a detail of another VCSELaccording to the invention, showing the thin slot combined with thethick slot and the use of an intra-cavity contact.

FIG. 13 shows calculated current injection profiles into the activeregion and three transverse optical modes.

FIG. 14 shows the calculated current-to-light characteristics for twoVCSELs showing the improved efficiency of the design according to theinvention.

FIG. 15 and 16 shows the modal gain of the three transverse modes shownFIG. 13 for the two calculated lasing characteristics in FIG. 14. FIG.15 shows the modal gains for the unimproved device. FIG. 16 shows themodal gains for the device according to the invention.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Referring to FIG. 3, there is shown a vertical-cavity surface emittinglaser device 100 in accordance with a specific embodiment of theinvention. The laser device 100 has a first electrode 12 disposed on topof a first mirror stack 14, a second electrode 16 disposed on asemiconductor substrate 18 beneath an active region 20 and a secondmirror stack 26 such that the active region, which is formed of aconventional InGaAs multiple quantum well, is contacted on opposingsides by the electrodes 12 and 16. The first mirror stack 14 defines awaveguide cavity which constrains the electromagnetic field and definesthe transverse optical mode 32. The axial mode of the electromagneticfield is defined by spacing of the reflective surfaces of the respectivemirror stacks and the thicknesses of the active region 20, the firstcontacting layer 22 and second contacting layer 24. The active region 20is ideally placed at the maximum in the axial mode's standing wavepattern and any optically-absorptive elements are disposed at nulls inthe standing wave pattern, as is understood in the art. The firstcontacting layer 22 is disposed above the active layer 20 in electricalcontact through the mirror stack 14 with the first electrode 12, and asecond contacting layer 24 is disposed below the active region inelectrical contact through mirror stack 26 with the second electrode 16.For laser operation under electrical excitation, it is required that oneof the contacting layers be p-type and that the other be n-type, as isunderstood in the art.

FIG. 4 shows the process whereby the current 30 is forced in towards thecenter of the cavity by an insulating slot 27. According to theinvention, the thickness of slot 27 is sufficiently thin that thetransverse optical mode 32 is not constricted, allowing alarger-diameter transverse optical mode and resulting in more efficientand single transverse-mode operation.

Referring to FIG. 4, an electrically-insulating slot 27 has been formedin the laser cavity which constricts the current to flow within anaperture identified as region 1. The modified cavity outside theconstriction is identified as region 2. Electrically-insulating slot 27could be formed by a selective technique such as wet etching or wetoxidizing a high aluminum containing semiconductor layer as is known inthe art. The high aluminum containing layer, such as AlAs, AlGaAs,AlInAsP or the like, is grown within the first contacting layer 22 andexposed by etching a mesa after epitaxial growth. In the InP system,highly selective wet etches exist for InGaAsP. Such techniques usuallyresult in an index of refraction associated with a vacuum or oxide, inthe range of 1 to 1.7, replacing the semiconductor index which isusually in the range of 3 to 3.6. Such a large change in index willcause large optical reflections, constrict the transverse optical mode32 and alter the optical path length between the first mirror stack 14and the second mirror stack 26. The change in optical path length willmanifest itself in a shift in the resonant wavelength of the cavity.This is undesirable and the shift should be minimized to achieve optimalsingle transverse mode performance. If the thickness of the slot 27 issufficiently thin, however, the phase of the reflections from the upperand lower interfaces will cancel and the slot will have little effect onthe optical properties of the cavity while the current 30 will remainconstricted.

In the preferred embodiment, the slot is placed near a null in the axialmode's standing wave pattern. Just as the optical losses are minimizedby placing the optically lossy materials near an axial standing-wavenull, so are the effect of index variations minimized near standing wavenulls. Referring to FIG. 5, the index of refraction and the axial mode'sstanding wave pattern 34 are shown in the vicinity of the active region20. The periodic steps in index on either side are the distributed Braggreflectors, with each layer a quarter of an optical wave in thickness asis standard in the art. The active region 20 shown in FIG. 5 comprises asingle 100 Å thick InGaAs quantum well. Many other active regions arepossible and are not limited by the invention. The plot corresponds toan axial cross section of region 2 in FIG. 4. The index of refraction ofthe electrically-insulating slot 27 is set at 1.0 as appropriate for avacuum. The axial mode is calculated using the transmission matrixtechnique as known in the art. The slot 27 is centered on a standingwave null and has been chosen to be 300 Å thick for this example. It isapparent that there are many other potential locations and thicknessesfor the slot (or slots) which would lie near standing wave nulls. It ispreferable to keep the slot 27 close to the active region to minimizethe spreading of the current 30 before it is injected into the activeregion 20. It is also preferable to place the current constricting slot27 on the p-type side of the active region 20 because of the lowerdiffusion constant of holes within the active region 20. A lowerdiffusion constant is preferable because the outward diffusion ofcarriers counteracts the current constricting effect of the slot 27.

Using the transmission matrix technique, the reflectivity of region Iand region 2 can be calculated. The inset in FIG. 6 shows a typicalreflectivity spectrum. The distributed Bragg reflectors, 14 and 26, havea limited band of high reflectivity. The resonance near the center isdetermined by the optical path length in the cavity and specifies thelasing wavelength λ_(c). The larger plot in FIG. 6 shows the detail ofthe cavity resonances for region 1 and region 2. The two resonances areshifted by an amount are equal to 0.5 nm. In the limit of small shifts,it is known that the transverse optical mode due to index guiding inVCSEL cavities can be calculated using: ##EQU1## where Δn is the changein effective index between region 1 and region 2 and n_(eff) is theeffective index of refraction. Using Eqn. (1) the transverse opticalmode can then be calculated once the reflectivity spectra in the tworegions are known. The criterion of just how thin theelectrically-insulating slot 27 must be can thus be quantified.

The transverse optical modes for a cylindrical waveguide of core indexn_(core) and cladding index n_(clad) =n_(core) -Δn are known as the HEand EH modes. The fundamental mode is HE₁₁ mode. The modes are typicallycharacterized by the radius ω_(o) at which the optical intensity hasdropped to (1/e)² or 0.135 times the intensity at the center. The beamdiameter 2ω_(o), also known as the beam waist, is plotted in FIG. 7 as afunction of the shift in cavity mode Δλ_(c) assuming a core diameter of5 μm, an effective index n_(eff) =3.25 and a cavity resonance λ_(c)=1000 nm. A dashed line indicates where the beam waist crosses fromregion 1 of FIG. 4 to region 2. It is apparent in FIG. 7 that as Δλ_(c)becomes less than 1 nm, or 0.1% of λ_(c), the transverse optical mode 32spreads out quickly. A fractional shift in the axial cavity mode'sresonance of less than 0.05% after formation of the slot is desirablefor single-mode operation. The preferred embodiment is to use another,larger-diameter index discontinuity to define the transverse opticalmode 32 and use Eqn. (1) to ensure that the current constricting slot 27is not the limiting factor. In this way the transverse optical mode 32is well defined and not dependent on subtle effects such as thermallyinduced index shifts which often vary with bias current.

From an optics point of view, it is desirable to make the slot 27 asthin as possible. From an electrical and technological point of view,thicker slots have higher breakdown voltages and are easier tofabricate. By placing the slot 27 near an axial-mode standing-wave null,thicker slots can be used. FIG. 8 shows an example of the shift incavity mode resonance Δλ_(c) for a VCSEL designed for operation at λ_(c)=1000 nm where an AlAs layer is converted into an air slot by selectiveetching. Two curves are shown, one with the slot centered on an axialstanding-wave null as in FIG. 5 and the other where the slot is centeredon an axial standing-wave peak. In both curves the slot 27 is placed inan Al₀.5 Ga₀.5 As cavity between 15 period distributed Bragg reflectorsof alternating AlAs and GaAs layers. Only the position within the cavityis changed. The preferred design of Δλ_(c) ≦0.5 nm requires a slotthickness of 300 Å or less if placed at a null but requires a slotthickness of 10 Å or less if placed at a peak. It is therefore mucheasier to realize constricted current confinement without constrictingthe transverse optical mode if the electrically-insulating slot 27 isplaced near an axial standing-wave null.

The detailed design shown in FIGS. 5-8 was used as an example toillustrate the effective implementation of the invention. The exact slotthicknesses and limitations will be different for the various materialsystems and lasing wavelengths but the principles and design techniqueremains the same. Many different embodiments are possible. FIG. 9 is aside cross-sectional view of a detail of another VCSEL according to theinvention, combining a thick electrically-insulating slot 38 defining alarger diameter constriction and a thin electrically-insulating slot 27defining a smaller diameter constriction. The thick slot 38 results in alarge axial cavity mode shift and thus confines the transverse opticalmode 32 while the thin slot 27 only constricts the current 30. Thisconfiguration has the advantage of a well defined mode diameter withmore efficient, single transverse-mode operation. FIG. 10 is a sidecross-sectional view of a detail of another VCSEL according to theinvention, showing a pair of thin insulating slots 27. Such aconfiguration has the advantage of minimizing outward carrier diffusionby constricting both electrons and hole injection into the active region20. FIG. 11 is a side cross-sectional view of a detail of another VCSELaccording to the invention, showing a thin slot 27 combined with a thickslot 38. This design has the same advantages as the embodiment of FIG. 9but can be easier to fabricate. Many selective techniques such as wetetching or wet oxidation become slow as the layer thickness becomes thindue to reactant transport limitations. By combining the thin slot 27with the thick slot 38, the thin slot need only extend 1-3 micronsbeyond the thick slot edge. The combined slot embodiment shown in FIG.11 thus removes reactant transport limits and facilitates theapplication of the selective techniques to mesas which are 10's of μm indiameter or more. As can be seen in FIG. 5, the axial mode's standingwave has a null at the outer edges of the low index layers comprisingthe mirror stacks. One particularly convenient arrangement, then, is touse the first low-index layer 36, within the mirror stack 14 or 26, forthe thick slot 38 and place the thin slot 27 on the edge away from theactive region 20. The thick slot 38 could be formed by making the firstlow-index layer 36 from Al_(y) Ga_(1-y) As and the thin slot 27 fromAl_(z) Ga_(1-z) As where z>y and where the remaining exposed low-indexlayers within mirror stacks 14 or 26 are made from Al_(x) Ga_(1-x) Aswhere y>x. The wet etch or oxidization rates increase with increasingaluminum content as is known in the art. The selectivity or variation inrate with aluminum content can be quite strong, and thus smalldeviations from design or lot to lot can significantly affect theprocess control. It can be preferable, then, to generate effectivealloys using digital superlattices of binary or ternary materials forrepeatable results.

FIG. 12 is a side cross-sectional view of a detail of another VCSELaccording to the invention, using a combined thick slot 38 and thin slot27 placed within the first contacting layer 22. An annular-like firstelectrode 12 has been placed in contact with the first contacting layer22. The first contacting layer 22 has been thickened to allow placementof the slots 27 and 38 in the proper location as shown for the low indexlayer 36 in FIG. 5. The structure shown in FIG. 12 has the advantage ofallowing insulating materials to be used for the first mirror stack 14.Other possible embodiments use several periods of conducting mirrorlayers and then an annular like electrode 12.

For effective implementation of the invention, one must also considerthe effects of current crowding and current spreading. Current crowdingoccurs around the edge of the electrically-insulating slot 27 becausethe current 30 finds the path of least resistance between the firstelectrode 12 and the second electrode 16. Because the currentconstriction is smaller than the contact diameters, the current willtend to crowd at the edges, particularly at higher bias currents whenthe junction resistance is low. Current spreading occurs as the currentspreads outward between the electrically insulating slot 27 and theactive region 20. Because the active region voltage drop is proportionalto the current density, a lower voltage drop and hence lower resistanceis encountered when the current passes through a larger active regionarea. This current spreading effect is particularly important at lowbiases, such as at lasing threshold, when the diode resistance is high.As disclosed in U.S. Pat. No. 5,343,487, the current crowding effect canbe minimized by introducing a resistive layer between the currentconstricting slot 27 and the active region 20. Current spreading is ofgreat concern in achieving low threshold lasing operation. Currentspreading can also be reduced by increasing the sheet resistance of thematerial between the current constricting slot 27 and the active region20. The sheet resistance is increased by reducing the material thicknessbetween the slot 27 and the active region 20. The sheet resistance isalso increased by reducing the mobility or concentration of carriers inthe material between the slot 27 and the active region 20. Referring toFIG. 13, the calculated current density profiles 32 injected into theactive region 20 for a range of bias currents are shown. Thecalculations are for a structure similar to FIG. 12 and are described ingreater detail in the Appendix. The dashed lines show the currentdensity profiles injected into the active region 20 assuming a currentconstriction diameter of 5 μm. Several current biases, found byintegrating the current density profiles over the active region, areindicated for a sense of scale. One can see a significant fraction ofthe current spreading outward at the low bias levels representative ofthe lasing threshold current. At higher biases, one can see the currentbegin to crowd around the current constriction, peaking at the edge andshowing lower injection levels in the center. Also shown in FIG. 13 arethree transverse optical modes defined by a large index discontinuity ata diameter of 9 μm. The fundamental transverse optical mode, the HE₁₁mode, has the best optical overlap with the injected current and thusthe laser will operate with a single transverse mode and with increasedefficiency compared with prior art which have the transverse opticalmode 32 and the diameter of current flow 30 coincident.

A laser according to the invention is compared with prior art lasers inFIGS. 14-16. Referring to FIG. 14, the calculated current-to-lightcharacteristics are shown for two VCSELs. All structural properties areidentical for the two calculations except the current constriction andtransverse mode diameters. The laser representative of prior art has a 7μm diameter current constriction and optical mode. The laser accordingto the invention has a 5 μm diameter current constriction defined by athin slot 27 and a 9 μm diameter mode defined by a thick slot 38. Thecurrent-to-light characteristics of the laser according to the inventionshow improved efficiency and improved linearity due to the improvedoptical overlap with the current 32 as shown in FIG. 13. Improvedefficiency and linearity are important benefits, particularly in analogcommunications applications.

Greater insight into the stabilization of the transverse optical mode bythe invention is gained upon reference to FIGS. 15 and 16. Bothcurrent-to-light calculations shown in FIG. 14 assume that only thefundamental transverse mode, the HE₁₁ mode, is lasing. There are anumber of other higher-order transverse modes which are well representedby the HE₂₁ and HE₁₂ modes shown in FIG. 13. It is desirable for manyapplications that the laser emit the optical power in a singletransverse optical mode. Such applications include laser printing,single mode fiber optic communication and sensors. During the course ofthe current-to-light curve, the carrier densities in prior art devicesrises at the edge of the optical mode due to the weak mode intensity.The rising carrier density at the periphery results in higher gain forthe higher-order transverse modes and generally results in laseroperation with multiple lateral modes. Such undesirable multimodeoperation can be avoided by the use of the invention. FIGS. 15 and 16show the round-trip modal gain for each of the three transverse opticalmodes during the course of the current-to-light curve of the prior artand laser according to the invention respectively. The gain of thelasing HE₁₁ mode is constant as is required. Referring to FIG. 15, theround-trip modal gain for the higher-order modes rise due to spatialhole burning. Higher optical losses for the higher-order modes maketheir threshold gain somewhat above the fundamental's threshold gain of1.53%. Without excessive additional optical losses, however, the laserwould be expected to have multimode operation at higher bias levels.This is the behavior observed for prior art VCSELs. Referring to FIG.16, the round-trip gain of the higher-order transverse modes neverexceed that of the fundamental, ensuring single transverse-modeoperation. Such stabilized single mode operation is a direct consequenceof constricting the current 30 to a diameter significantly less than thetransverse optical mode 30, thereby eliminating spatial hole burningeffects.

In the above examples cylindrical geometry was assumed and the specificcalculations were for an AlGaAs VCSEL comprising InGaAs quantum wells inthe active region 20. The physical principles underlying the inventionare very general and are equally applicable to other geometries andmaterial systems including square or rectangular constrictions andphosphide and nitride based semiconductors. The invention is applicableto VCSELs emitting in the infrared, visible and ultraviolet wavelengths.The invention is applicable not only to electrically-pumped but also tooptically-pumped VCSELs. The structure shown in FIG. 10 is particularlyuseful for optically pumped VCSELs, as both electrons and holes can begenerated on either side of the active region 20. The embodiment can beextended to optically pumped VCSELs with multiple active regions byplacing pairs of slots 27 around each active region 20.

The invention has now been explained with reference to specificembodiments. Other embodiments will be apparent to those of ordinaryskill in the art upon reference to this disclosure. It is therefore notintended that this invention be limited, except as indicated by theappended claims.

What is claimed is:
 1. A vertical-cavity surface emitting laser device(VCSEL) comprising:an active region; first and second mirror stacksforming a resonant cavity, said first mirror stack placed above saidactive region and said second mirror stack placed below said activeregion, at least one of which allows the partial emission of the laserlight; first and second electrodes disposed so as to cause a currentflow through said active region; a first contacting region and a secondcontacting region on each of a first and second side of said activeregion in electrical contact with said respective first electrode andsaid second electrode, each one of said contacting regions providing acurrent path for distributing current through the active region; and athin dielectric slot forming a current aperature that is non-conductiveplaced above said active region for guiding current flowing through theactive region to an area near the center of the active region, said slotbeing sufficiently thin so as not to constrict a fundamental opticalmode of said VCSEL.
 2. The device according to claim I wherein thecurrent aperature formed by said slot is less than the (1/e)² diameterof the transverse optical mode thereby improving device efficiency. 3.The device according to claim 1 wherein the current aperature formed bysaid slot is less than the (1/e) diameter of the transverse optical modethereby improving device efficiency and ensuring single mode operation.4. The device according to claim 1 wherein said slot has been placednear an axial standing wave null in said resonant cavity.
 5. The deviceaccording to claim 2 wherein said slot has been placed near an axialstanding wave null in said resonant cavity.
 6. The device according toclaim 3 wherein said slot is placed near an axial standing wave null insaid resonant cavity.
 7. The device according to claim 1 furthercomprising multiple dielectric slots.
 8. The device according to claim 2further comprising multiple dielectric slots.
 9. The device according toclaim 3 further comprising multiple dielectric slots.
 10. The deviceaccording to claim 1 wherein said slot comprises a thin part forming anarrow aperture and not constricting the optical mode and a thick partforming a wider aperture which defines the transverse optical mode. 11.A vertical-cavity surface emitting laser device (VCSEL) comprising:anactive region; first and second mirror stacks forming a resonant cavity,said first mirror stack placed above said active region and having onesurface through which laser light may be emitted and said second mirrorstack placed below said active region; firsthand second electrodesdisposed so as to cause a current flow through said active region; afirst contacting region and a second contacting region on each of afirst and second side of said active region in electrical contact withsaid respective first electrode and said second electrode, each one ofsaid contacting regions providing a current path for distributingcurrent through the active region; and a thin dielectric insulating slotformed in said first contacting region forming a current aperature forguiding current flowing through the active region to an area near thecenter of the active region, said slot being sufficiently thin so as notto constrict a fundamental optical mode of said VCSEL.
 12. The deviceaccording to claim 11 further comprising a second dielectric insulatingslot above said first slot, said second slot being thicker than saidfirst slot in order to more effectively block current and said secondslot extending less towards a center of said VCSEL cavity than saidfirst slot so as to not interfere with the optical mode of said VCSEL.13. The device according to claim 11 further comprising a second thindielectric insulating slot below said active region so to include twocurrent aperatures.
 14. The device according to claim 11 wherein saidthin slot is an extension of a thicker slot, said thicker slot extendingtowards a center of said VCSEL cavity and defining a larger constrictionthan said thin slot, said thin slot extending further towards a centerof said VCSEL cavity than said thick slot wherein the thick slotconfines the transverse optical mode and the current while the thin slotconstricts the current only.
 15. The device according to claim 14wherein said thin slot is placed near an axial standing wave null insaid resonant cavity.
 16. The device according to claim 14 wherein saidthin slot is manufactured subsequent to said thick slot using aselective techniques whereby said manufacturing is facilitated becausesaid thin slot need extend only a small distance from the end of saidthick slot.
 17. The device according to claim 11 wherein an axial modeis defined by the spacing of the reflective surfaces of the said firstand second mirror stacks and by the thicknesses of said active regionand said first and second contacting layers and wherein said activeregion is placed at a maximum in the axial mode's standing wave pattern.18. The device according to claim 11 wherein one of said first andsecond contacting layers is a p-type contacting layer and the other isan n-type contacting layer and wherein said slot is placed on the p-typelayer side of the active region so as to restrict hole current flow. 19.The device according to claim 11 wherein a larger-diameter indexdiscontinuity is used to define the transverse optical mode and theequation ##EQU2## is used to ensure that the slot is not the limitingfactor.
 20. The device according to claim 11 wherein said slot is placedat an axial mode standing wave null and has a thickness between 250 and350 Å.
 21. The device according to claim 11 wherein said thick slot isformed within a first low-index of one of said mirror stacks and saidthin slot is placed on the edge of said thick slot away from said activeregion.
 22. The device according to claim 21 wherein said thick slot isformed by making said first low-index layer from Al_(y) Ga_(1-y) As andsaid thin slot 27 is formed from Al_(z) Ga_(1-z) As where z>y and wherethe remaining exposed low-index layers within said mirror stacks aremade from Al_(x) Ga_(1-x) As where y>x.
 23. The device according toclaim 11 wherein a combined thick slot and thin slot are placed withinsaid first contacting layer and wherein said first electrode is anannular-like electrode placed in contact with said first contactinglayer and said first contacting layer is thickened to allow placement ofthe combined slots in the proper location for the low index layerallowing insulating materials to be used for the first mirror stack. 24.The device according to claim 11 further comprising a nonlinearincreasing resistivity profile in at least one of said contactingregions between said active region and said slot where said increasingresistivity is such that the resistivity adjacent to said active regionis at least one order of magnitude greater than the resistivity incontact with said first or second electrode to minimize current crowdingat perimeter edges of said active region.